The field of this disclosure relates generally to detection systems for use in imaging systems, and more specifically, to solid-state photomultipliers having a wide temperature range of operation in a detection system.
Many known imaging detector systems use a solid-state photomultiplier (SSPM) in combination with a scintillator. The scintillator converts x-rays and gamma rays to visible light photons whereupon a photodiode of the SSPM converts these photons to photocurrent. SSPMs have come to replace photomultiplier tubes (PMTs) in the field due to the SSPM's lower manufacturing costs, lower power requirements, compact size, mechanical durability, insensitivity to magnetic fields, and uniformity of response. SSPMs have become widely used as detection elements for computed tomography (CT), single photon emission computed tomography (SPECT), and positron emission tomography (PET) machines and applications. SSPMs have also become known for gamma ray detection in oil and gas drilling applications. High energy gamma rays reflected from Hydrogen (H) bearing compounds underground are indicative of petroleum-containing specific locations.
Most known SSPMs, however, are temperature dependent. Specifically, the typical SSPM integrates a dense array of small avalanche photodiodes (APD) operating in Geiger mode, i.e., well above the avalanche breakdown voltage. Each APD element in the array is often referred to as a “microcell”, or “pixel”, and each microcell has its own quenching resistor. The breakdown voltage (Vbr) of the microcell is the bias point at which the electric field strength generated in a depletion region of the microcell is sufficient to create an avalanche breakdown. Each microcell generates a highly uniform and quantized amount of charge every time the microcell undergoes a avalanche breakdown. The breakdown voltage, though, changes as a function of temperature. That is, the gain of a given microcell depends at least in part on temperature. Thus, variations in gain due to temperature effects may lead to uncertainty with respect to the magnitude of the actual underlying event being measured.
To compensate for this temperature dependence, some known SSPMs require a cooling system, such as an additional Peltier device, to cool and regulate the temperature of the SSPM. Other known SSPMs utilize additional circuitry to adjust the bias voltage to maintain a constant over voltage with respect to the breakdown voltage. All of these solutions require additional hardware, which can increase the cost of the device. Additionally, even with this additional hardware, the typical SSPM is limited to moderate temperature environments, e.g., a hospital room with a PET scanner, where the SSPM can be easily cooled and/or regulated. The temperature dependence of an SSPM has made it difficult to utilize SSPMs in applications where a wide range of operational temperatures are experienced, such as with the gamma rays and neutron radiation detected in oil exploration drilling, for example, or the measurement of the flame from a gas turbine engine, which typically exhibit significantly harsher thermal environments. A typical oil well environment, for example, can experience shock levels near 250 times gravitational acceleration (G), and temperatures therein vary widely from below room temperature to above 175 degrees Celsius (° C.).